1. Technical Field
Disclosed embodiments concern plasma etch processes employed in etching dielectric layers on a workpiece such as a semiconductor wafer.
2. Background Art
Plasma etching of high aspect ratio dielectric structures on workpieces is hampered by problems of aspect ratio dependent etching, etch stop or twisting of etched features. This is due in part to competing polymer deposition and etch mechanisms using fluorocarbon etch chemistry. It is also due in part to negative charging of the structure walls and positive charging of the structure floor or bottom of the high aspect ratio openings from predominantly positive ion fluxes and unequal electron fluxes down the depth of the structure. There is a need to provide for more balanced positive and negative charging of the structure walls being etched to avoid an accumulation of a net charge.
A pulsed plasma is used in etching of non-dielectric materials (such as polysilicon) using electronegative species in the process gas, such as chlorine. Generally, pulsing of the plasma reduces the effective electron temperature. Plasma source power and bias power may be synchronously pulsed (same pulse repetition frequency) with pulses in phase (overlapping RF envelopes) or with phase lead or lag of the source and bias RF envelopes. To minimize charging effects of the workpiece structure where the material is not a good conductor, source and bias power may be pulsed synchronously, but with the bias “on” envelope applied when the source power is “off”. The time window during which the pulsed source power is “off” may be referred to as the “decay phase” or the pulsed source power “off” phase. In a polysilicon plasma etch process using a process gas with electronegative chemistry such as Cl2, negatively charged Cl ions may form during the “decay phase” when the source power is “off”. Under appropriate conditions, the number density of negative ions may greatly exceed the number density of electrons, and be nearly equal to the number density of positive ions that form during the source power “on” phase. If the bias power is applied during the source power “off” phase, then the workpiece being etched may be bombarded with nearly equal fluxes of positive and negative ions during the two phases. Specifically, the workpiece is bombarded with predominantly positive ions during the source power “on” phase and with negative ions during the bias power “on” phase (i.e., source power decay or “off” phase). If the structure being etched is bombarded with alternating (per RF cycle) equal fluxes of oppositely charged (˜equal mass) ions, then no net charging (time average) of the structure bottom should occur.
Etching of dielectric materials such as silicon dioxide selective to mask layers or underlayers is typically performed using a fluorocarbon chemistry, such as CF4 or C4F6, in conjunction with an oxidizer such as O2 or CO, and sometimes hydrogen or hydrocarbon chemistry as well. Typically, an electropositive gas such as Ar is added as a diluent. Unfortunately, the typical dielectric etch process gas chemistries do not form a significant population of negative ions; that is, the number density of electrons far exceeds the number density of negative ions. Consequently, there are unequal fluxes of oppositely charged ions to the workpiece or wafer, leading to charging of the workpiece structure. Pulsing the plasma decreases plasma-on time, which can limit at least to some degree the charge damage, because the collapsed sheath allows for neutralization of accumulated charge. However, that is not a complete solution to the charging problem. Moreover, it does not address a problem of non-uniform radial distribution of plasma ion density.